This invention pertains to audio signal equalizers including tone controls of the type that provide alterable amplitude versus frequency responses in selected bands of the audio spectrum. As an example, the equalizer may take the form of simple tone controls in which bass (low frequencies) and/or treble (high frequencies) are controllably boosted or cut to the listener's satisfaction.
Tone control equalizers, as the term is used here, refers to relatively uncomplex bass and treble tone controls found on most high-fidelity systems, and on certain professional audio recording counsels and mixers. The tone control circuitry that is typically found in such systems most commonly uses a negative feedback circuit of the type disclosed by P. J. Baxandall based on work done in 1952. Baxandall, P. J., "Negative Feedback Tone Control--Independent Variation of Bass and Treble Without Switches", W.W. 58.10 (October 1952) 402; Correction 58.11, (November 1952) 444. The Baxandall tone control circuit is commonly referred to as a "shelving" control because of the shape created by the amplitude versus frequency response when boosting or cutting the low and high end frequencies. The shape of the response curves when using such a "shelving" circuit is that of a shelf, contrasted with peak or dip-type response shapes, and further contrasted with total lifting or rolling off of the response. That is to say, the shelving tone controls cause amplification (boost) or attenuation (cut) at a substantially constant slope or rate and then level off to a flat response.
Almost universally, these shelving tone controls are provided by one-pole filter circuits. The steepest response slope that can ever be achieved by a one-pole filter is a 6 dB/octave (corresponds also to 20 dB/decade). This would be for an ideal filter circuit. In practice, the overall shelving tone control transfer function results in a response slope of the tone control that rarely exceeds about 3 dB/octave. This is a relatively gentle slope and causes the control to influence the mid-band frequencies because of overlapping effects that extend into the response characteristics of the mid-band from the adjacent high and low frequencies. Such corruption of the mid-band frequencies causes disturbing effects on the critical mid-band frequencies whenever bass or treble is altered by relative boosting or cutting.
Ideally, the boost and cut tone controls should change the slope of the transition frequencies into the high and low end but should not alter the response characteristics of the center or mid-band frequencies. Practical, existing filter designs have thusfar not effectively isolated the high and low band tone controls from the center or mid-band frequencies.
Although it is conceivable that filters having additional poles with sharper response characteristics could be incorporated into the tone control circuitry, such multiple pole filters create a different problem. That problem is the creation of excessive phase shift, approaching 180.degree., that produces cancellations and dropouts due to frequencies that are out of phase with the input signal. Stability problems also appear because of the tendency of oscillations at or near the 180.degree. phase shift regime. Rather than accept such problems of dropouts, or cancellations and other instabilities, most tone control circuits use the single pole filter. Such single pole filters have a maximum phase shift of 90.degree. and, therefore, are inherently stable and preclude cancellations since the frequency shift never approaches the 180.degree. regime.